Thermochemical Assessment of the Aromatic and Antiaromatic Characters of the Cyclopropenyl Cation, Cyclopropenyl Anion, and Cyclopropenyl Radical: A High-Level Computational Study

Abstract

The aromatic stabilization energy of the cyclopropenyl cation (CH)3+ is assessed with G2 theory by calculating its homodesmotic stabilization energy (247.3 kJ mol-1) and by comparing the ionization energies of the cyclopropenyl radical (6.06 eV) and the cyclopropyl radical (8.24 eV). These data indicate substantial stabilization of the two π-electron system in what is considered the archetypal aromatic cation. The calculated enthalpy of formation of the cyclopropenyl cation is 1074.0 kJ mol-1 and agrees with the experimental estimate of 1075 kJ mol-1. The small stabilization energy of the cyclopropenyl radical (37.4 kJ mol-1) suggests that this radical should not be classified as aromatic, in contrast to earlier suggestions. Our G2-calculated enthalpy of formation of the cyclopropenyl radical (ΔHf 298 = 487.4 kJ mol-1) and its ionization energy are different from experimental estimates and suggest that the experimental values may need to be revised. The most stable structure for the cyclopropenyl anion is...